我到处寻找，教授给我的建议并不像我希望的那么明确。我正在尝试将某个对象添加到队列中，并且我不断收到NPE错误。有趣的是，当我有队列或队列时，队列工作得很好，排队工作正常。当我尝试使用我的特定对象类型Queue时，它只会出现问题。你们可以查看我的代码并尝试提供帮助吗？
我的老师让我确保我已经为队列分配了内存，并且我检查队列是否为空，但我确定我正在做两件事。
Queue.java -
package queues;
import java.util.Iterator;
public class Queue<Type> implements Iterable {
private String name;
private Node head, tail;
private int size;
//basic constructor for the queue
public Queue (String name){
System.out.println("Constructing queue");
this.name = name;
this.size = 0;
}
//front of the list = end of the queue
public boolean enqueue(Type data){
System.out.println("Enqueue");
if(this.isEmpty()){
Node newNode = new Node(data);
this.head = newNode;
this.tail = newNode;
this.size++;
return true;
}
this.head = this.head.insert(data);
this.size++;
return true;
}
public Type dequeue(){
Node temp = this.head;
while(temp.next!= this.tail){
temp = temp.next;
}
this.tail = temp;
temp = temp.next;
this.tail.next = null;
this.size--;
return temp.getData();
}
public boolean isEmpty(){
return this.size == 0;
}
//returns the current size of the queue
public int size(){
System.out.println("getting queue size");
return this.size;
}
// console display for testing
public String toString(){
Node current = this.head;
String retString;
retString ="[ ";
for(int i = 0 ; i < this.size; i++){
if(i != 0){
retString += ",";
}
retString += current.getData();
current = current.getNext();
}
retString += " ] ";
return retString;
}
public Type peek(){
if(this.size == 0){
return null;
}
return this.tail.getData();
}
public String getName(){
return this.name;
}
private class Node {
// data
private Node next;
private Type data;
// constructors
public Node(Type data){
this.next = null;
this.data = data;
}
public Node(Type data, Node next) {
this.next = next;
this.data = data;
}
// inserts new nodes to the front of the list aka the top of the stack
public Node insert(Type data) {
return new Node(data,this);
}
// returning the next member
public Node getNext() {
return next;
}
// helps with peek
public Type getData() {
return this.data;
}
public String toString(){
return this.data.toString();
}
}
public java.util.Iterator<Type> iterator(){
return new QueueIterator();
}
private class QueueIterator implements Iterator<Type>{
private Node currentNode;
public QueueIterator(){
currentNode = head;
}
public boolean hasNext() {
return currentNode == null;
}
public Type next() {
if(!hasNext()){
System.out.println("Stack Empty");
return null;
}
Type temp = currentNode.getData();
currentNode = currentNode.next;
return temp;
}
}
}
这是我尝试入队的类，我收到了错误
//Jukebox.java
package queues;
import java.io.*;
import java.util.*;
import cs1c.*;
public class Jukebox {
private Queue<SongEntry> favoritesPL, loungePL, roadTripPL ;
//Constructor for Jukebox initializes the 3 queues
public Jukebox(){
System.out.println("Constructing Jukebox");
Queue<SongEntry> favoritesPL = new Queue<SongEntry>("favorites");
Queue<SongEntry> loungePL = new Queue<SongEntry>("loungePL");
Queue<SongEntry> roadTripPL = new Queue<SongEntry>("roadTripPL");
System.out.println(favoritesPL.toString());
}
//Reads the given file and searches allSongs and if the song is found, it is added
// to the correct playlist
public void fillPlaylists(String requestFile, SongEntry[] allSongs){
ArrayList<String> tempList = new ArrayList<String>();
try{
Scanner scanner = new Scanner(new File(requestFile));
//seperates the info by commas and newLines
scanner.useDelimiter(",|\\n");
while(scanner.hasNext()){
tempList.add(scanner.next());
}
scanner.close();
}
catch(Exception e){
System.out.println("File Not Found");
}
/* Tests the output of the tempList
for(int i = 0; i < tempList.size(); i++){
System.out.println(tempList.get(i));
}
*/
for(int i = 1; i < tempList.size(); i++){
// for(int j = 0; j < 5000; j++){
System.out.println("i = " + i );
SongEntry tempSongEntry = search(tempList.get(i),allSongs);
System.out.println(tempSongEntry);
System.out.println("Returned from search");
//if the song is not found, don't add null to the playlist
if(tempSongEntry != null){
System.out.println("Song Entry not null");
switch(tempList.get(i-1)){
// This is where I keep getting my error.
case "favorites": favoritesPL.enqueue(tempSongEntry);
break;
case "lounge": System.out.println("About to Enqueue");
loungePL.enqueue(tempSongEntry);
System.out.println(loungePL.toString());
break;
case "road trip": roadTripPL.enqueue(tempSongEntry);
break;
default: break;
// }
}
}
}
}
//helper method to search the json file and return the Song Entry if found
public SongEntry search(String songName, SongEntry[] allSongs) {
System.out.println("Searching");
for(int i = 0; i < allSongs.length; i++){
if(allSongs[i].getTitle().equals(songName)){
System.out.println("Found + "+ i);
// loungePL.enqueue(allSongs[i]);
return allSongs[i];
}
}
return null;
}
//Accessors
public Queue<SongEntry> getFavoritePL(){
return this.favoritesPL;
}
public Queue<SongEntry> getLoungePL(){
return this.loungePL;
}
public Queue<SongEntry> getRoadTripPL(){
return this.roadTripPL;
}
}
答案 0 :(得分：2)
您的类级别队列永远不会被初始化，因为您在构造函数中声明了新的队列。
更改为：
public Jukebox(){
System.out.println("Constructing Jukebox");
this.favoritesPL = new Queue<SongEntry>("favorites");
this.loungePL = new Queue<SongEntry>("loungePL");
this.roadTripPL = new Queue<SongEntry>("roadTripPL");
System.out.println(favoritesPL.toString());
}